This invention relates to a circuit arrangement for generating a pulsatory supply voltage for a load from a direct voltage, in which the predetermined effective value of the supply voltage remains substantially constant in the case of a varying direct voltage, said circuit arrangement comprising a switch supplying the supply voltage, a low-pass filter coupled to the load and generating a mean value voltage, and a Schmitt trigger circuit maintaining the mean value voltage and closing the switch when the mean value voltage reaches a first switching threshold and opening the switch when the men value voltage reaches a second switching threshold.
Such a circuit arrangement is known from DE-OS 33 08 959 which is used for generating, from a fluctuating direct voltage, a pulsatory supply voltage whose effective value is substantially constant. The output of an ohmic load or the light output of an incandescent lamp is directly proportional to the effective value of the supply voltage which is present. In the known circuit arrangement an incandescent lamp is fed via a switch from an accumulator or a battery whose direct voltage decreases during operation. The supply voltage is squared in a squaring member and the squared supply voltage is compared with a nominal voltage in a summing device. The output voltage of the summing device is applied via a low-pass filter, which generates a mean value voltage, to a Schmitt trigger circuit which controls the switch arranged between the direct voltage source and the incandescent lamp. The nominal voltage corresponds to the squared predetermined effective value. The switching thresholds of the Schmitt trigger circuit are adjusted in such a way that the ratio between pulse duration and pulse interval (pulse duty factor) corresponds to the mean value of the output voltage of the summing device. The squaring member, which is required in the circuit, generally has many components, is complicated and is not stable as regards temperature without an additional temperature compensation circuit.